Research

A Molecular Synchrotron

The combination of Stark-decelerated beams with molecular storage rings offers interesting prospects for molecular scattering studies. In its simplest form, a storage ring is a trap in which the molecules -rather than having a minimum potential energy at a single location in space- have a minimum potential energy along a circle. The first storage ring for neutral polar molecules was devised by bending a long electrostatic hexapole into a torus. As a result of the longitudinal velocity spread of the packet of molecules, however, the packet gradually spreads out along the ring on making successive round trips.

To take full advantage of the ring structure for scattering experiments, the molecules should remain in a bunch as they revolve around the ring. For this, a new ring consisting of 2 half rings separated by a 2 mm gap was constructed. Every time the molecules pass through the gap, the electric fields are switched such as to focus the molecules in the longitudinal direction. Using this bunching scheme, the spreading out of the molecules is counteracted. Bunching ensures a high density of stored molecules, and in addition makes it possible to inject multiple -either colinear or counterpropagating- packets of molecules into the ring without affecting the packet(s) that are already stored. While circling the ring, these packets can be made to interact repeatedly at well defined times and at distinct positions.

The number of packets that can be stored simultaneously, and the efficiency of the bunching process, depend on the number of individual segments of the synchrotron. We have recently developed a molecular synchrotron that consists of 40 short hexapole segments. This synchrotron enables the storage of up to 20 packets of molecules simultaneously, that are kept together as a compact bunch for a distance of over a mile. We are currently setting up a second injection beam-line to study collisions between counter-propagating packets of molecules in the synchrotron.

These experiments with molecular storage rings have been the thesis work of Floris Crompvoets and Cynthia Heiner, and have been supervised by Dr. Rick Bethlem (now at the Vrije Universiteit Amsterdam). Since March 2008 the synchrotron project is, in a close collaboration with Rick, embedded in our group. Click here to visit Rick's research homepage in Amsterdam!